Over the years, cam actuated fuel injectors have become increasingly complex in a search for ever expanding performance capabilities. The same is true for other types of fuel injectors including hydraulically actuated and common rail injectors with admission valves. In general, a fuel injection system with a broader range of capabilities is able to increase engine performance while at the same time reducing undesirable exhaust emissions, including particulate matter, unburned hydro-carbons, NOx, etc. One of the first innovations in improving the capabilities of cam actuated fuel injectors was to include an electronically controlled spill valve. This innovation is shown in many prior art references and allowed for some independence in injection timing from that dictated by a rotating cam lobe whose position was generally fixed with respect to the engine's crank shaft. Much later, a newer innovation was included that provided direct control over the injector's needle valve, to open and close the nozzle outlets at a selected timing that was somewhat independent of the pressurized state of the fuel injector.
For instance, co-owned U.S. Pat. No. 5,551,398 to Gibson et al. teaches a cam actuated fuel injector with electronic control over both pressurization via an electronically controlled spill valve and electronic control over injection timing via a separate needle control valve. Directly controlled fuel injectors generally have a needle valve that includes a closing hydraulic surface exposed to fluid pressure in a needle control chamber. A separate electronically controlled needle control valve can be actuated or deactuated to change the pressure conditions in the needle control chamber. When pressure is high in the needle control chamber, the needle stays in, or moves toward, its closed position. When pressure is low in the needle control chamber, the needle will lift to its open position, provided that fuel in the injector is above a needle valve opening pressure that can overcome a spring bias tending to hold the needle valve member in its closed position. This reference teaches a typical aspect of the conventional wisdom with regard to directly controlling needle valves in that steps are taken to minimize the volume of the needle control chamber in order to increase fluid tightness in the control circuit and hasten the needle's response to the control valve's movement. In other words, because fuel is not incompressible, there must inherently be some delay when raising the pressure in the needle control chamber to compress the fluid therein. As a consequence of this volume minimizing strategy, the needle's biasing spring must often be located at a different location outside of the needle control chamber. While the fuel injector taught in this reference shows considerable promise, it includes an increased complexity and part count in order to produce its superior performance.
Another cam actuated fuel injector is taught in U.S. Pat. No. 5,893,350 to Timms. This reference teaches the use of a single electrical actuator to control both pressurization through a spill valve and needle control via a needle control valve. While this fuel injector deletes one electrical actuator, it inherently couples injection timing to fuel pressurization and also suffers from an inability to do substantial end of injection rate shaping, which is more recently becoming recognized as a means by which emissions can be further reduced. In other words, this injector shows little ability to control the fuel pressure at the timing in which the needle valve closes at the end of an injection event.
The present invention is directed to an improved compromise between cost, complexity and part count on one hand and performance capabilities on the other hand.